The development of DSL technology is aimed at a constantly increasing signal bandwidth. In the VDSL2 (“Very high-speed DSL”) standardization process, a maximum transmission bandwidth of 30 MHz beyond the bandwidth already known from the ADSL (“Asymmetric DSL”) standard is intended. The development of analogue front ends for such bandwidths is generally a challenge, since the performance requirements for VDSL2 are based on the same model of the line noise at −140 dBm/Hz as that used for narrower bandwidths, such as ADSL (1.1 MHz), ADSL2+ (2.2 MHz), SHDSL (600 kHz) (“Symmetric High-Bitrate DSL”) or VDSL1 (12 MHz). Echo compensation filters are also employed, for example, in xDSL transceivers and there particularly in the analogue front end, in order to replicate the echo component caused by the transmission signal in the reception signal so that the transmission signal component in the reception signal can then be reduced.
Such electrical echoes often occur in so-called “4-wire-to-2-wire” hybrid circuits, since these cannot achieve complete separation of the reception and transmission paths. The conventional methods are based on a linear model for replicating the echo.
In VDSL, most echo signal components are compensated for by VDSL techniques, for example by FDD (“Frequency Division Duplex”) and digital duplex. Even here, however, the nonlinear echo components cause a deterioration of the performance features. On the one hand, the out-of-band distortion caused by nonlinearities influences the reception signal and, on the other hand, the in-band nonlinearities influence the signal-to-noise ratio at the far end.
Nonlinear echo components can be reduced by using nonlinear echo compensators. These use adaptive methods based on memoryless and memory-related models for the nonlinearities. Adaptive polynomial characteristics inter alia are used as an approach for the former case and adaptive Volterra filters in the second case. Since in particular the use of Volterra filters entails a very large number of system parameters to be modeled, the efficient realization of Volterra filters is still in development.
FIG. 4 represents a method and a device for echo compensation according to the prior art. In this case, a transmission signal s is delivered to a line driver 1, and a reception signal r is applied via a transmitter 3. The line driver and the transmitter are respectively connected to ohmic terminating impedances 5a, 5b connected in series. On the line driver-output side, the amplified transmission signal is tapped and delivered to an echo compensation filter 4, which replicates the echo component caused by the transmission signal s in the reception signal r. An echo compensation signal e determined in this way and the reception signal r tapped from the transmitter 3 are delivered to a combination means 2, which may be designed as a differential amplifier, so as to obtain an echo-compensated reception signal r′ with a reduced echo component, to which end the echo compensation signal e is subtracted from the reception signal r.
Owing to a large bandwidth in xDSL applications, and particularly in VDSL2 applications, the problem may arise that the signal is not merely delivered via a signal path through the transceiver but distributed between a plurality of signal paths. To this end, a separate echo compensation filter must be provided in each individual signal path, which entails increased material and production costs as well as a higher power consumption of the overall component.